Method and apparatus for developing electrostatic images

ABSTRACT

A method and apparatus to develop electrostatic images using a mist which is substantially water and a colorant. Mist is transported from a generation station to a development station via a development electrode. Unused mist may be recycled or extracted. The mist may be size classified before development so that a consistent and desired resolution is obtained. Classification by size may be achieved by passing the mist in an arcuate path from the generation station to the development station.

TECHNICAL FIELD

This invention relates to a method of and an apparatus for developingelectrostatic images and particularly, to the development of such imagesby means of generated finite marking entities which are based on waterand hence are environmentally friendly.

BACKGROUND ART

In the process of xerography, as disclosed in Carlson U.S. Pat. No.2,297,691, issued Oct. 6, 1942, a xerographic plate comprising a layerof photoconductive insulating material on a conductive backing is givena uniform electric charge over its surface and is then exposed to thesubject matter by conventional projection techniques. This exposuredischarges the plate areas in accordance with the radiation intensitythat reaches them, and thereby creates an electrostatic latent image onor in the photoconductive layer. Development of the latent image isusually effected with an electrostatically charged, finely-dividedmaterial such as an electroscopic powder that is brought into surfacecontact with the photoconductive layer and is held thereonelectrostatically in a pattern corresponding to the electrostatic latentimage. The developed electrostatic image is usually transferred to asupport surface to which it is fixed by any suitable means. Where theelectrostatic plate comprises a consumable material the developed imagemay be fixed directly.

Alternatively, the recording member may be a dielectric, as disclosed inBeaudet et al, U.S. Pat. No. 4,413,049 issued Nov. 1, 1983, where thedielectric recording member comprises sealed porous anodised aluminium.The recording member is charged with an electrostatic image, as sodescribed in Fotland et al, U.S. Pat. No. 4,267,556 issued May 12, 1981,by a glow discharge electron source. Development of the so formed latentimage is again effected with an electrostatically charged,finely-divided material such as an electroscopic powder that is broughtinto surface contact with the dielectric surface and is held thereonelectrostatically in a pattern corresponding to the electrostatic latentimage. The developed latent image is usually transferred to a supportsurface to which it is fixed by any suitable means. Where the dielectricrecording member comprises a consumable material the developed image maybe fixed directly. Various development systems are known in the artincluding cascade, powder cloud, liquid, magnetic, brush and the like,each of which employ electroscopic marking particles that arepreferentially charged to a polarity. The charged particles arepresented to the latent image to develop it. One common drawback of manyof said development systems is dust generation which further requiresthe necessity to contain such dust.

In U.S. Pat. No. 2,690,394 to Carlson issued 28 Sept., 1954 there isdescribed a system of electrostatic imaging which utilises an atomiserto produce a spray of droplets of a solvent. The droplets of solvent arecharged by an electrode and then are drawn by means of a suction pumpover a recording surface including an electrostatic image. Droplets ofsolvent are attracted by the oppositely charged image and deposit on therecording surface. The solvent is then transferred from the recordingmember to a sheet of paper and the paper then passes over a dye coatedroller so that some of the dye is dissolved onto the paper to produce animage. Solvents proposed include high boiling point alcohols,cellosolve, toluene, cyclohexanol acetate and alcohol-water mixtures. Itis also suggested that mixtures of solvent with an ink or dye can beatomised and also that the solvent can contain suspended pigments andbinders. There is no disclosure, however, of a liquid system where theliquid is composed primarily of water.

It has been disclosed in Heine-Geldern et al, U.S. Pat. No. 3,795,443,issued Mar. 5, 1974, that some liquids not always characterised as beingelectrically resistive when generated into a fine mist in the vicinityof an electrostatic charge pattern will deposit selectively onto anelectrostatic image. By chemically colouring the liquid, as by dyes andpigments the selective deposition produces an image reproduction whichoptionally can subsequently be transferred from or fixed directly on theimage bearing surface. The development by this means is effected withoututilising a carrier. Such a system experiences difficulty in developingsolid areas without the benefit of external control means such as adevelopment electrode.

Such methods as disclosed by Heine-Geldern et al and Carlson arecharacterised by inadequate development of the electrostatic image bymists such as ultrasonically generated liquid mists. Typically, onlysmall amounts of colourant, be it dye stuff or pigment, deposit onto anelectrostatic image on the recording member, be it photoconductor ordielectric, in a reasonable time, with respect to a practical device foroffice or industrial reproduction. Thus commercialisation of such anapparatus, as disclosed in Heine-Geldem et al and Carlson is seen asdifficult with regard to the excessively long time that is required fordevelopment of the latent image, notwithstanding the low quantity ofcolouring material which as a result is deposited onto the recordingmember during such time.

Indoor air quality in offices and other areas where electrostatic imagedevelopment is used can be seriously affected by hydrocarbon solventswhich characterise many liquid development systems and dust whichcharacterises many powder development systems. There is considerableadvantage if an electrostatic image development system could be providedwhich uses water as the main carrier rather than a hydrocarbon solvent.

It is therefore an object of the invention to provide novel method fordeveloping electrostatic latent images using water based developmentsystems, including those pertaining to dielectric recording members.

It is a further object of the invention to provide a method by whichgenerated liquid mists substantially of water are effective fordeveloping electrostatic latent images by way of their movement betweena development electrode and such latent images.

DISCLOSURE OF THE INVENTION

In accordance with this instant invention it has been discovered thatsome liquids characterised by being electrically conductive, such aswater, when existing as a fine liquid mist generated by some mechanicalmeans, can be transported to an electrostatic charge pattern such thatthe particles of the mist, will selectively, instantly deposit on suchpattern thereby effecting rapid development. As the electricallyconductive liquid may contain a colourant, either dye or pigment or acombination thereof the selective deposit thus produced can besubsequently transferred from or fixed to the image bearing surface, asan image reproduction.

Development by this method does not necessarily require the externalcharging of the liquid mist, however, it has been observed that chargingof the liquid mist, enhances latent image development if the image is ofopposite polarity, whilst a latent image of the same polarity, willactively repel said mist, further, a latent image containing bothpolarities will develop a deposit of excellent integrity, in that, socalled background fog is eliminated.

Alternatively the invention provides a novel method in which generatedliquid mists comprising substantially water are effective for developingelectrostatic latent images by way of their movement between such latentimage and a development electrode, which may be in the form of forinstance a roller positioned and made to rotate, to aid and control themovement of said liquid mist, thereby enhancing the development of saidlatent image. Droplets of such a generated liquid mist may attain acharge, by some external means, and thereby are effective for developingelectrostatic latent images. As is also known to those skilled in theart, the charge on the droplets of said liquid mist may be of either apositive or negative nature depending on the charge of the said latentimage and the desired form of the so developed latent image.Alternatively, generated liquid mists, composed of, by and large,uncharged droplets, are effective for developing electrostatic latentimages by way of their movement between such latent image and adevelopment electrode or the like, positioned at some distance from thesaid latent image, as is known to those skilled in the art.

Alternatively the invention provides a novel method in which generatedliquid mists, composed of charged droplets of water, are effective fordeveloping electrostatic latent images by way of their movement betweensuch latent image and a development electrode, in the form of a roller,positioned and made to rotate, to aid and control the movement of saidliquid mist, thereby enhancing the development of said electrostaticlatent image. The development electrode, may impart charge onto dropletsof said liquid mist, as well as to aid and control the movement of suchdroplets in said liquid mist, thereby further enhancing the developmentof said electrostatic latent image.

In one form therefore the invention is said to reside in a method ofdeveloping a latent electrostatic image comprising the steps of;

(a) producing a mist from a liquid comprised substantially of water witha colourant incorporated therein such that the colourant is supported inthe mist,

(b) transporting the mist to a developer station,

(c) passing the mist between a development electrode and a recordingmember incorporating the electrostatic image such that its direction oftravel is substantially tangential or parallel to the recording member,

(d) providing an electric field between the development electrode andthe recording member, and

(e) attracting the mist by means of the electric field to theelectrostatic image to thereby develop the electrostatic image.

In an alternative form the invention may be said to reside in a methodof developing a latent electrostatic image comprising-the steps of;

(a) producing a mist from a liquid comprised substantially of water witha colourant incorporated therein such that the colourant is supported inthe mist at a production station,

(b) transporting the mist to a developer station,

(c) passing the mist between a development electrode and a recordingmember incorporating the electrostatic image at the development stationsuch that the direction of travel of the mist is substantiallytangential or parallel to the recording member,

(d) providing an electric field between the development electrode andthe recording member at the development station, and

(e) attracting the mist by means of the electric field to theelectrostatic image to instantaneously develop the electrostatic image.

In an alternative form the invention may be said to reside in anelectrostatic image development arrangement comprising;

(a) means to produce a mist from a liquid comprised substantially ofwater with a colourant incorporated therein such that the colourant issupported in the mist,

(b) means to transport the mist to a development station,

(c) means to transport a recording member having a latent electrostaticimage thereon through the development station,

(d) the means to transport the mist further transporting the mist acrossthe recording member between the recording member and an adjacentdevelopment electrode such that its direction of travel is substantiallytangential or parallel to the recording member, and

(e) means to apply a electric field between the development electrodeand the recording member whereby the mist is attracted to theelectrostatic image to thereby develop it.

In an alternative form the invention may be said to reside in anelectrostatic image development arrangement for high speed printingcomprising;

(a) mist production means to produce a mist from a liquid comprisedsubstantially of water with a colourant incorporated therein such thatthe colourant is supported in the mist,

(b) means to transport the mist to a development station in an arcuatepath,

(c) means to transport a recording member having a latent electrostaticimage thereon through the development station,

(d) the means to transport the mist further transporting the mist acrossthe recording member between the recording member and an adjacentdevelopment electrode such that its direction of travel is substantiallytangential to the recording member, and

(e) means to apply a electric field between the development electrodeand the recording member whereby the mist is attracted to theelectrostatic image to instantaneously develop it.

In an alternative form the invention may be said to reside in anelectrostatic image development arrangement for high speed printingcomprising;

(a) mist production means to produce a mist from a liquid comprisedsubstantially of water with a colourant incorporated therein such thatthe colourant is supported in the mist,

(b) means to transport the mist to a development station in an arcuatepath,

(c) means to transport a recording member having a latent electrostaticimage thereon through the development station,

(d) the means to transport the mist further transporting the mist acrossthe recording member between the recording member and an adjacentdevelopment electrode such that its direction of travel is substantiallyparallel to the recording member, and

(e) means to apply a electric field between the development electrodeand the recording member whereby the mist is attracted to theelectrostatic image to instantaneously develop it.

The colourant supported in the mist may also include bactericides,humectants, dispersants, fixing agents, binders and charge controlagents such that all of these may be supported in the mist and becarried with the mist to deposit on the electrostatic image.

In a preferred embodiment of the invention the droplets of the generatedliquid mists, effective for developing electrostatic latent images maycontain a colourant in the form of a dye stuff or mixture of dye stuffs,a pigment or mixture of pigments or mixtures of dyestuffs and pigmentsso as to render the said latent electrostatic image visible whendeposited thereon. Alternatively the liquid may include a polymericmaterial which upon deposition onto the recording member surface may befixed such that the recording member may be used as a plate for offsetprinting without an image being visible thereon.

The mist may be transported in an arcuate path between the mistproduction station and the development station. In one embodiment thearcuate path may be defined between two curved plates. The flow of mistin the arcuate path may be caused by the spray generation of the mist orit my be induced by forced removal of mist beyond the developmentstation. Alternatively the arcuate path may be provided by the misttravelling around part of the surface of a drum which comprises thedevelopment electrode.

Preferably the development electrode is a drum adapted to rotate and themist is carried around adjacent the periphery of the drum from a mistgeneration station to the development station in a flow of air caused bythe movement of the drum. The drum may have a surface which hassufficient surface roughness to carry the mist between the point ofgeneration and the development station. The surface of the drum may bealuminium with a grained aluminium oxide coating so that the surfacedoes not discharge the charged mist. The development electrode drum maybe rotated at a speed to give a surface speed which is considerablyhigher than the surface speed of the recording member through thedevelopment station. The development electrode drum may be rotated so asto give a surface speed of from 1 to 10 meters per second.

The means to produce a mist of droplets may produce droplets having asize range of from less than 0.1μ to greater than 100μ. It is desirablefor good resolution of a developed image that the droplets not have toowide a size range. If the droplets all have the same charge and too widea size range then they will give an uneven image when deposited onto therecording member. One preferred method of selecting mist droplets of adesired size is to transport them in an arcuate path between theproduction station and the development station so that the dropletswhich are larger and hence more massive will move by their momentumtowards the outer curve of the arcuate path and may be removed by someform of baffle. Droplets which are smaller and hence lighter will notmove so far away from the drum and may not be close enough to therecording member at the development station to take part in thedevelopment of the image. Intermediate sizes of droplets may be selectedfor development of the electrostatic image

There may be a shroud surrounding but spaced from the periphery of thedrum. The mist may be retained in the annular space between the shroudand the periphery of the drum by means of an electric field betweenthese members counteracting the motion of the droplets of the mistbetween the development electrode and the shroud due to their momentum.The momentum of the droplets provides a substantially tangential motionat any point thus tending to move them in a straight line which willmove them nearer the shroud.

The action of the classification of the mist droplets may be enhanced orachieved by the use of an electric field between the developmentelectrode and the shroud. This can be achieved by applying differentvoltages to the development drum and the shroud. The developmentelectrode may have a polarity opposite to that of the mist droplets sothat they are attracted to the development electrode. Movement of thedroplets in the arcuate path, however, will provide a momentum on thedroplets tending to move them away from the development electrode. Fordroplets of a desired selected charge and mass the force ofelectrostatic attraction and the motion due to their momentum willbalance each other and no net motion of the droplets relative to thedrum surface will occur. Where the size and hence mass is high comparedto the charge the motion due to their momentum will overcome theelectrostatic force and droplets will impinge onto the shroud. Suitablecollection means may be provided to remove such impinged droplets. Wherethe size is smaller then the electrostatic force may be higher than themotion due to their momentum and such droplets will be attracted to thedrum and not take part in the development process.

Droplets may be classified, therefore, by size and speed of travel inthe arcuate path such as by selection of the surface speed of thedevelopment electrode drum. Further classification may be achieved bythe voltage difference between the shroud and the development electrode.It may be desirable to use both forms of classification as the use ofelectric field classification only may not remove larger multiplecharged droplets.

Classification of droplets according to size may not be necessary if amethod of droplet formation is used which does not give a wide range ofdroplet sizes.

The recording member may be another drum or a surface on another drumand hence the development station may be the region of closest proximitybetween the two drums. Alternatively the recording member may be aplanar surface transported through the development station. Recordingmembers to which the present invention is applicable may include a drumwith a photoconductive surface, a drum with a dielectric surface or aconsumable material such as paper or an offset printing plate on whichthe image is developed.

The mist may be generated by a variety of means, including the use ofultrasonic transducers, high pressure spray and the like. Droplet sizeof the order of 0.1 to 100 microns are useable; 0.5 to 5.0 microns arepreferred.

The mist may be charged either to a positive or negative charge eitherat or immediately after generation.

Unused mist may be extracted when the mist has passed the developmentstation and this may be done for instance by a vacuum extractor.

Alternatively the mist may be reused by recirculating the mist on thedevelopment drum past the mist generation to again enter the developmentstation. There may be provided an auxiliary fan or other air circulatingmeans to assist with recirculating of the mist the amount ofrecirculation of mist may be controlled to give a steady flow of mist ora desired mist density through the development station.

In an alternative form the invention may be said to reside in a methodof developing a latent electrostatic image for high speed printingcomprising the steps of;

(a) producing a mist from a liquid comprised substantially of water witha colourant incorporated therein such that the colourant is supported inthe mist,

(b) transporting the mist to a developer station in an arcuate path,

(c) passing the mist between a development electrode and a recordingmember having a surface incorporating the electrostatic image at thedevelopment station such that the direction of travel of the mist issubstantially parallel to the surface of the recording member,

(d) providing an electric field between the development electrode andthe recording member,

(e) attracting the mist by means of the electric field to theelectrostatic image to instantaneously develop the electrostatic image,and

(f) extracting the residual unused mist from the development station.

In an alternative form the invention may be said to reside in anelectrostatic image development arrangement for high speed printingcomprising;

(a) mist production means to produce a mist from a liquid comprisedsubstantially of water with a colourant incorporated therein such thatthe colourant is supported in the mist,

(b) means to transport the mist in an arcuate path to a developmentstation,

(c) classification means to select mist droplets of a selected sizerange and to discard droplets above and below the selected size range,

(d) means to transport a recording member having a latent electrostaticimage thereon through the development station,

(e) the means to transport the mist further transporting the mist of theselected size range across the recording member between the recordingmember and an adjacent development electrode such that its direction oftravel is substantially parallel or tangential to the recording member,

(f) means to apply an electric field between the development electrodeand the recording member whereby the mist is attracted to theelectrostatic image to instantaneously develop it, and

(g) means to extract unused mist from the development station.

In an alternative form the invention may be said to reside in a methodof developing an electrostatic image using a mist comprisingsubstantially of water with a colourant incorporated therein such thatthe colourant is supported in the mist including the step of classifyingthe mist into a selected droplet size range before development of theimage.

In an alternative form the invention may be said to reside in anelectrostatic image development arrangement for high speed printingincluding mist production means to produce a mist from a liquidcomprised substantially of water with a colourant incorporated thereinsuch that the colourant is supported in the mist and classificationmeans to separate droplets from the mist which are above or below aselected size range before development of an electrostatic image.

According to this invention a high speed printer may be one adapted toprint an image onto a recording member such as a paper at speed of up toa half or one meter per second or up to 100 pages per minute.

This then generally describes the invention but to assist withunderstanding of the various features, advantages and limitations of theinvention reference will now be made to the following description anddrawings of preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a schematic first embodiment of an apparatus for developingan electrostatic image according to this invention,

FIG. 2 shows a schematic second embodiment of an apparatus fordeveloping an electrostatic image according to this invention,

FIG. 3 shows a schematic third embodiment of an apparatus for developingan electrostatic image according to this invention,

FIG. 4 shows a schematic fourth embodiment of an apparatus fordeveloping an electrostatic image according to this invention, and

FIG. 5 shows an alternative embodiment of apparatus for effectingdevelopment in accordance with this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows an embodiment of the present invention in schematic form.The positioning of the various components does not necessarily indicatethe desired positions in a production model of such a machine.

The apparatus for developing an electrostatic image comprises a watermist generation device 1 adapted to direct mist into the arcuate space 2between a development drum 3 and a shroud 4. The development drum 3 isadapted to rotate as indicated by the arrow so as to draw the mistaround in the arcuate path. The mist is transported to a developmentstation 5 which is the region of nearest proximity between thedevelopment drum 3 and a recording drum 6. Unused mist is carriedfurther around the development drum 3 to an extraction duct 7.Extraction of unused mist is enhanced by an extraction fan 8. The mistmay be condensed and the waste liquid passed to storage (not shown) orrecycled to the water mist generation device. The recording drum mayhave a recording member on its surface upon which an electrostatic imageis produced at imaging station 9 and which is then rotated in thedirection of the arrow to the development station for instantaneousdevelopment of the image. The imaging station 9 may include means tocharge the recording member and form the required electric chargepattern as is well known in the art. Deposited image fixing orsubsequent transferral of the image (not shown) after the developmentstation will be well understood by those skilled in the art.

FIG. 2 shows a further embodiment of this instant invention. A rotatingrecording member 11 at a potential V₁ being of dielectric nature, has anelectric charge pattern deposited onto the surface of said recordingmember by an array of computer controlled glow discharge electronsources 12. The recording member is rotated so that the electric chargepattern is brought into contact with a liquid mist 13 at a developmentstation 10. The liquid mist 13 is transported to the development stationby way of a cylindrical development electrode 14, to which a potentialV₂ is applied. The liquid mist is made to impinge upon the cylindricaldevelopment electrode 14 by an ultrasonic atomiser 15 at potential V₃which generates the mist and passes it through a charging tunnel 19which is at a potential V₅ and onto the development electrode 14. Theink is supplied by tube 17 to the atomiser 15 and the position of theatomiser 15 is such that the particles of the mist are attracted to thedevelopment electrode 14 and then to the latent image electric chargepattern on said recording member 11. The mist is contained by a shroud16 at potential V₄ as it is transported to the development station, withthe excess droplets of said liquid mist being removed by vacuum suctionmeans 18 after the development station.

The mist as it travels around the development electrode 14 travels in anarcuate path so that the motion and momentum of the droplets tends tocause the droplets to travel tangentially in a straight line away fromthe development electrode. This motion due to momentum is balanced by anelectric field force generated by the difference in the voltages V₄ andV₂ so that droplets which have a selected charge/mass ratio will betransported to the development station 10. Droplets with a highercharge/mass ratio will be drawn to the development electrode 14 andthose with a lower charge/mass ratio will move out to the shroud 16.This provides classification of droplets to a selected size and hencewill assist in providing an even resolution on the developed image.

FIG. 3 shows another embodiment of this instant invention. A rotatingelectrophotographic recording member 21 at a potential V₁ is uniformlycharged by a corona 22, exposed to reflected light through a lens 30from an illuminated copyboard 31, thereby selectively discharging saidrecording member in all areas subjected to light and so forming a latentimage. The recording member 21 is brought into contact with the liquidmist 23, by way of a rotating cylindrical development electrode 24, towhich a potential V₂ is applied. The liquid mist is generated by anultrasonic atomiser 25 which is at a potential V₃ which is supplied withink by tube 27. The position of the atomiser 25 is selected such thatthe droplets of said mist are attracted to said developing electrode 24and then to the latent image on said recording member. The mist iscontained by shroud 26, at potential V₄. Excess particles of the liquidmist are carried around by the drum 24 to a centrifugal fan 35 whichassists with recirculation of the mist back to the development stationthereby providing for recycling of the mist. This allows a closed orsubstantially closed system thereby giving minimum wastage ofconsumables and a system which does least damage to indoor air quality.

Once again as the mist travels around the development electrode 24 ittravels in an arcuate path defined on the outside of the curve by theshroud 26 so that the momentum of the droplets in the mist tends to movethem outwards. This momentum is balanced by an electric field forcegenerated by the difference in the voltages V₄ and V₂ so that dropletswhich have a selected charge/mass ratio will be transported to thedevelopment station 23. Droplets with a higher charge/mass ratio will bedrawn to the development electrode 24 and those with a lower charge/massratio will move out to the shroud 26. This provides classification ofdroplets to a selected size and hence will assist in providing an evenresolution on the developed image.

FIG. 4 shows yet another embodiment of this instant invention. Arotating recording member 41, at a potential V₁ being of dielectricnature, has an electric charge pattern deposited onto the surface ofsaid recording member by an array of computer controlled glow dischargeelectron sources 42. The recording member is brought into contact withthe liquid mist 43, by way of a cylindrical development electrode 44, towhich a potential V₂ is applied and onto which the said liquid mist ismade to impinge by a pressure spray 45, which is at a potential V₃. Theink is supplied by a pump (not shown) through a tube 47 and the airpressure by a compressor (not shown) through line 49. The position ofsaid pressure spray is selected such that the particles of said mist areattracted to said developing electrode 44 and then to the latent imageon said recording member 41. The said mist is contained by shroud 46, atpotential V₄, with the excess droplets of said liquid mist being removedby vacuum suction means 48. In this embodiment a degree of recycling isused. The recycling can be controlled by the use of the vacuum suctionmeans 48.

FIG. 5 shows yet another embodiment of this instant invention. Anelectrophotographic plate 51, being of finite length or continuous andwhich may be of the consumable type, at a potential V₁, is uniformlycharged by a corona 52, exposed to reflected light through a lens 60,from a illuminated copyboard 61, thereby selectively discharging saidrecording member in all areas subjected to light and so forming a latentimage. The recording member is brought into contact with the liquid mist53 at a development station 57. The liquid mist 53 is transported to thedevelopment station 57 by way of a cylindrical development electrode 54,to which a potential V₂ is applied and onto which the liquid mist ismade to impinge after being generated by an ultrasonic atomiser 55,which is at a potential V₃. The ink from which the mist is generated issupplied by tube 59. The position of the atomiser is such that theparticles of said mist are attracted to said developing electrode 54 andthen to the latent image on said electrophotographic plate 51 by beingcontained by shroud 56, at potential V₄. The excess droplets of saidliquid mist are removed by vacuum suction means 58. In this embodiment adegree of recycling is used. The recycling can be controlled by the useof the vacuum suction means 58.

The droplets of the liquid mist are preferably of such dimension thatthey exhibit a volume resistivity and dielectric constant which in air,does not substantially reduce said latent image electrostatic field.Mist may be generated by a variety of means, including the use ofultrasonic transducers, high pressure spray and the like. Droplet sizeof the order of 0.1 to 100 microns are useable, 0.5 to 5.0 microns arepreferred. The droplets may be charged but this is by no means anessential requirement of this invention. However, it has been found thatimage quality is a function of both the droplet charge distribution anddroplet size distribution. As those skilled in the art would recognise,a narrow distribution of droplet charge, including the condition of nilcharge, yields superior image quality than that obtained with a widedistribution of said droplet charge. Likewise with respect to dropletsize, a narrow distribution is associated with excellent image quality,especially with regard to resolution and background fog. Fastdevelopment of the latent image has been found to be effected when boththe droplet size distribution and charge distribution is narrow.

The developing electrode, as described in the above embodiments, is inthe form of a cylinder or roller which is effective in transporting thedroplets of mist to the recording member such that instant developmentoccurs. As is known to those skilled in the art, a development electrodeenhances the electrostatic field emanating from a recording member, thuseffecting improved so called fill-in of the developed image. In thisinvention, the development electrode acts both to aid the deposition ofthe marking particles in the form of droplets and to transport the mistof said droplets to the latent image such that instant development isexpedited. It has been found that the surface speed of said developmentelectrode cylinder affects the efficiency of the development of thelatent image. For instant development of said recording member, surfacespeeds of 0.1 to 10 meters per second are suitable but 1 to 5 meters persecond are preferred. Other characteristics of said developmentelectrode which affect development include surface topology, surface andvolume resistivity. It has been found that a grained metal surface ispreferred for instant development of the latent image but thisperception should not be interpreted as restricting the scope of thisinstant invention. By applying appropriate electric potentials V₁ and V₂during latent image development, high optical image density with verylow background fog can be easily achieved. In the preferred embodimentof this invention the development electrode is at ground potential withV₁ between 50 to 100 volts, although image deposition has been observedwith voltages in the range 0 to 500 volts.

The droplets constituting the mist may acquire a charge by some externalmeans which may include the employment of a charge tunnel 19 as in FIG.2 or the like but notwithstanding inductive charging by the applicationof appropriate potential difference between the atomiser or spray nozzle45 and the developing electrode 44 (FIG. 4), or likewise between theenvelope 16, 26, 46, 56 and the developing electrode 14, 24, 44, 54 orindeed between the recording member 11, 21, 41, 51 and the developingelectrode 14, 24, 44, 54 The preferred embodiment is with a potentialapplied between the pressure spray nozzle and the developing electrode,where the magnitude of this potential is between 500 and 5000 volts,with the preferred range being between 1000 and 2000 volts. Neverthelesshigh optical density images have been observed with no external chargingof the droplets in the said mist.

The position of the vacuum suction means, envelope and developingelectrode should be juxtaposed such that an appropriate flow of mistcontaining droplets is rendered at the latent image to expedite instantdevelopment of said latent image. The positions shown in the variousembodiments illustrated are schematic only.

The intensity of the suction means should be such that the mist ofdroplets can fully develop the latent image to a high optical density,without depositing in the non-image or background areas so as to producebackground fog. As will be known to those skilled in the art, the vacuumsuction means could be replaced by a positive displacement system suchas a fan or the like, so as to propel the mist of droplets to the latentimage. This arrangement has certain advantages with respect torecirculating the mist of droplets thus producing a so called closeddeveloping system, eminently suitable with respect to an environmentwhere indoor air quality standards must be complied with.

The following examples are provided by way of illustration of severalembodiments of the invention.

EXAMPLE 1

A dielectric drum comprising of a sealed porous anodised aluminiumsurface was used in an apparatus as disclosed in FIG. 2.

The dielectric recording member was rotated while an electric chargepattern was deposited onto the surface of said recording member by anarray of computer controlled glow discharge electron sources.

The recording member at ground potential was rotated at 0.2 ms⁻¹ suchthat the electric charge pattern was brought into contact with theliquid mist.

The liquid mist being generated by an ultrasonic atomiser at groundpotential and through a charging tunnel at a potential of -2000 volts.

The mist was contained by the shroud which was at 100 volts potential.

The liquid mist was transported to the development station by way of thecylindrical development electrode rotating at 2.0 ms⁻¹ and which was at150 volts potential.

An image of good resolution and density was obtained by this method.

EXAMPLE 2

A dielectric drum comprising of a DuPont Teflon FEP film coated surfacewas used in an apparatus as disclosed in FIG. 2.

The dielectric recording member was rotated while an electric chargepattern was deposited onto the surface of said recording member by anarray of computer controlled glow discharge electron sources.

The recording member at 50 volts potential was rotated at 0.2 ms⁻¹ suchthat the electric charge pattern was brought into contact with theliquid mist.

The liquid mist was generated by an ultrasonic atomizer at groundpotential and through a charging tunnel at a potential of -2000 volts.

The mist was contained by the shroud which was at ground potential.

The liquid mist was transported to the development station by way of thecylindrical development electrode rotating at 2.0 ms⁻¹ and which was atground potential.

A dense image of good resolution was obtained by this method.

EXAMPLE 3

A dielectric drum comprising of Tetlight TCF polyester film by Oike &Co. Ltd. was used in an apparatus as disclosed in FIG. 2.

The dielectric recording member was rotated while an electric chargepattern was deposited onto the surface of said recording member by anarray of computer controlled glow discharge electron sources.

The recording member at 0 volts potential was rotated at 0.2 ms⁻¹ suchthat the electric charge pattern was brought into contact with theliquid mist.

The liquid mist being generated by an ultrasonic atomizer at groundpotential and through a charging tunnel at a potential of -2000 volts.

The mist was contained by the shroud which was at -100 volts potential.

An image of very good resolution was obtained by this method.

EXAMPLE 4

An OPC drum was used in an apparatus as disclosed in FIG. 3.

The recording member was rotated while being uniformly charged by acorona, exposed to reflected light through a lens from an illuminatedcopyboard, thereby selectively discharging said recording member in allareas subjected to light and so forming a latent image.

The recording member at ground potential was rotated at 0.2 ms⁻¹ suchthat the latent image was brought into contact with the liquid mist.

The liquid mist being generated by an ultrasonic atomizer at a potentialof 2000 volts.

The mist was contained by the shroud which was at ground potential.

The liquid mist was transported to the development station by way of thecylindrical development electrode rotating at 2.0 ms⁻¹ and which was atground potential.

An image of good resolution and density was obtained by this method.

EXAMPLE 5

A dielectric drum comprising of Tetlight TCF polyester film by Oike &Co. Ltd was used in an apparatus as disclosed in FIG. 4.

The dielectric recording member was rotated while an electric chargepattern was deposited onto the surface of said recording member by anarray of computer controlled glow discharge electron sources.

The recording member at -100 volts potential was rotated by 0.5 ms⁻¹such that the electric charge pattern was brought into contact with theliquid mist.

The liquid mist being generated by a 150 μm nozzle pressure spray of 75kPa at 3000 volts potential.

The mist was contained by the shroud which was at 200 volts potential.

The liquid mist was transported to the development station by way of thecylindrical development electrode rotating at 5.0 ms⁻¹ and which was at300 volts potential.

An image of good density and resolution was obtained by this method.

EXAMPLE 6

A dielectric drum comprising of Tetlight TCF polyester film by Oike &Co. Ltd was used in an apparatus as disclosed in FIG. 4.

The dielectric recording member was rotated while an electric chargepattern was deposited onto the surface of said recording member by anarray of computer controlled glow discharge electron sources.

The recording member at -100 volts potential was rotated at 0.5 ms⁻¹such that the electric charge pattern was brought into contact with theliquid mist.

The liquid mist being generated by a 150 μm nozzle pressure spray of 150kPa at 3000 volts potential.

The mist was contained by the shroud which was at 200 volts potential.

The liquid mist was transported to the development station by way of thecylindrical development electrode rotating at 5.0 ms⁻¹ and which was at300 volts potential.

An image of high density and good resolution was obtained by thismethod.

EXAMPLE 7

A zinc oxide electrophotographic plate was used in an apparatus asdisclosed in FIG. 5.

The recording member was rotated while being uniformly charged by acorona, exposed to reflected light through a lens from an illuminatedcopyboard, thereby selectively discharging said recording member in allareas subjected to light and so forming a latent image.

The recording member at -50 volts potential was rotated at 0.3 ms⁻¹ suchthat the latent image was brought into contact with the liquid mist.

The liquid mist being generated by an ultrasonic atomizer at 3000 voltspotential.

The mist was contained by the shroud which was at 100 volts potential.

The liquid mist was transported to the development station by way of thecylindrical development electrode rotating at 5.0 ms⁻¹ and which was at150 volts potential.

An image of good density and resolution was obtained by this method.

Throughout this specification and the claims that follow unless thecontext requires otherwise, the words `comprise` and `include` andvariations such as `comprising` and `including` will be understood toimply the inclusion of a stated integer or group of integers but not theexclusion of any other integer or group of integers.

We claim:
 1. A method of developing a latent electrostatic imagecomprising the steps of;(a) producing a mist from a liquid comprisedsubstantially of water with a colourant incorporated therein such thatthe colourant is supported in the mist, (b) transporting the mist to adeveloper station in an arcuate path, (c) passing the mist between adevelopment electrode and a recording member incorporating theelectrostatic image such that its direction of travel is substantiallytangential or parallel to the recording member, (d) providing anelectric field between the development electrode and the recordingmember, and (e) attracting the mist by means of the electric field tothe electrostatic image to thereby develop the electrostatic image.
 2. Amethod of developing a latent electrostatic image comprising the stepsof;(a) producing a mist from a liquid comprised substantially of waterwith a colourant incorporated therein such that the colourant issupported in the mist at a production station, (b) transporting the mistto a development station in an arcuate path, (c) passing the mistbetween a development electrode and a recording member incorporating theelectrostatic image at the development station such that the directionof travel of the mist is substantially tangential or parallel to therecording member, (d) providing an electric field between thedevelopment electrode and the recording member at the developmentstation, and (e) attracting the mist by means of the electric field tothe electrostatic image to instantaneously develop the electrostaticimage.
 3. A method as in claim 2 further comprising classifying the mistinto a selected size range before transporting it to the developmentstation.
 4. A method as in claim 2 wherein the development electrode isa first drum adapted to rotate and the mist is carried in the arcuatepath around the periphery of the drum from the production station to thedevelopment station.
 5. A method as in claim 4 wherein the first drumhas a surface which has sufficient surface roughness to assist withcarrying the mist between the production station and the developmentstation.
 6. A method as in claim 5 wherein the first drum is rotated ata speed to give a surface speed of from 1 to 10 meters per second.
 7. Amethod as in claim 4 wherein the first drum has a surface of aluminiumwith a grained aluminium oxide coating such that the surface does notdischarge the mist.
 8. A method as in claim 4 wherein the first drum isrotated at a speed to give a surface speed which is considerably higherthan the surface speed of the recording member through the developmentstation.
 9. A method as in claim 2 wherein the recording member is adrum or mounted onto a drum and the development station is the region ofclosest proximity between the second drum and the development electrode.10. A method as in claim 9 wherein the drum has a photoconductivesurface.
 11. A method as in claim 9 wherein the drum has a dielectricsurface.
 12. A method as in claim 2 wherein the recording memberincludes a planar surface which is adapted to be transported through thedevelopment station.
 13. A method as in claim 2 wherein the recordingmember is a consumable material comprising paper or an offset printingplate on which the image is developed.
 14. A method as in claim 2further including a shroud surrounding but spaced from the periphery ofthe development electrode and wherein the mist is retained in the spacebetween the shroud and the development electrode by means of an electricfield between these members counteracting the momentum of the dropletstending to move them away from the development electrode caused by thearcuate path of the mist.
 15. A method as in claim 2 wherein the mist isgenerated by ultrasonic transducers or high pressure spray.
 16. A methodas in claim 2 wherein the mist has droplet size of from 0.1 to 100microns.
 17. A method as in claim 16 wherein the mist has a droplet sizefrom 0.5 to 5 microns.
 18. A method as in claim 2 wherein the mist ischarged either to a positive or negative charge either at or immediatelyafter the time of production.
 19. A method as in claim 2 wherein unusedmist is extracted when the mist has passed the development station. 20.A method as in claim 2 wherein the mist is reused by recirculating themist to the development station.
 21. A method as in claim 2 wherein thecolourant comprises a dye stuff or mixture of dye stuffs, a pigment ormixture of pigments or a mixtures of dyestuffs and pigments so as torender the said latent electrostatic image visible when depositedthereon.
 22. A method as in claim 2 wherein the colourant comprises apolymeric material which upon deposition onto the recording membersurface may be fixed such that the recording member may be used as aplate for offset printing without an image being visible thereon.
 23. Amethod as in claim 22 wherein the colourant further includesbactericides, humectants, dispersants, fixing agents, binders and chargecontrol agents such that all of these may be supported in the mist andbe carried with the mist to deposit on the electrostatic image.
 24. Amethod of developing a latent electrostatic image for high speedprinting comprising the steps of;(a) producing a mist from a liquidcomprised substantially of water with a colourant incorporated thereinsuch that the colourant is supported in the mist, (b) transporting themist to a development station in an arcuate path, (c) passing the mistbetween a development electrode and a recording member having a surfaceincorporating the electrostatic image at the development station suchthat the direction of travel of the mist is substantially parallel ortangential to the surface of the recording member, (d) providing anelectric field between the development electrode and the recordingmember, (e) attracting the mist by means of the electric field to theelectrostatic image to instantaneously develop the electrostatic image,and (f) extracting residual unused mist from the development station.25. An electrostatic image development arrangement comprising;(a) meansto produce a mist from a liquid comprised substantially of water with acolourant incorporated therein such that the colourant is supported inthe mist, (b) means to transport the mist to a development station, (c)means to transport a recording member having a latent electrostaticimage thereon through the development station in an arcuate path, (d)the means to transport the mist further transporting the mist across therecording member between the recording member and an adjacentdevelopment electrode such that its direction of travel is substantiallytangential or parallel to the recording member, and (e) means to applyan electric field between the development electrode and the recordingmember whereby the mist is attracted to the electrostatic image tothereby develop it.
 26. An electrostatic image development arrangementfor high speed printing comprising;(a) mist production means to producea mist from a liquid comprised substantially of water with a colourantincorporated therein such that the colourant is supported in the mist,(b) means to transport the mist to a development station in an arcuatepath, (c) means to transport a recording member having a latentelectrostatic image thereon through the development station, (d) themeans to transport the mist further transporting the mist across therecording member between the recording member and an adjacentdevelopment electrode such that its direction of travel is substantiallyparallel to the recording member, and (e) means to apply an electricfield between the development electrode and the recording member wherebythe mist is attracted to the electrostatic image to instantaneouslydevelop it.
 27. An electrostatic image development arrangement as inclaim 26 wherein the recording member includes a planar surface which isadapted to be transported through the development station.
 28. Anelectrostatic image development arrangement for high speed printingcomprising;(a) mist production means to produce a mist from a liquidcomprised substantially of water with a colourant incorporated thereinsuch that the colourant is supported in the mist, (b) means to transportthe mist to a development station in an arcuate path, (c) means totransport a recording member having a latent electrostatic image thereonthrough the development station, (d) the means to transport the mistfurther transporting the mist across the recording member between therecording member and an adjacent development electrode such that itsdirection of travel is substantially tangential to the recording member,and (e) means to apply a electric field between the developmentelectrode and the recording member whereby the mist is attracted to theelectrostatic image to instantaneously develop it.
 29. An electrostaticimage development arrangement as in claim 28 wherein the developmentelectrode is a first drum adapted to rotate and to carry the mist in thearcuate path around the periphery of the first drum from the mistproduction means to the development station.
 30. An electrostatic imagedevelopment arrangement as in claim 29 wherein the first drum has asurface which has sufficient surface roughness to assist with carryingthe mist between the mist production means and the development station.31. An electrostatic image development arrangement as in claim 29wherein the drum has a surface of aluminium with a grained aluminiumoxide coating such that the surface does not discharge the charged mist.32. An electrostatic image development arrangement as in claim 29wherein the first drum is rotated at a speed to give a surface speedwhich is considerably higher than the surface speed of the recordingmember through the development station.
 33. An electrostatic imagedevelopment arrangement as in claim 29 wherein the first drum is rotatedso as to give a surface speed of from 1 to 10 meters per second.
 34. Anelectrostatic image development arrangement as in claim 28 wherein therecording member is a drum or mounted onto a drum and the developmentstation is the region of closest proximity between the drum and thedevelopment electrode.
 35. An electrostatic image developmentarrangement as in claim 34 wherein the drum has a photoconductivesurface.
 36. An electrostatic image development arrangement as in claim34 wherein the drum has a dielectric surface.
 37. An electrostatic imagedevelopment arrangement as in claim 28 wherein the recording member is aconsumable material comprising paper or an offset printing plate onwhich the image is developed.
 38. An electrostatic image developmentarrangement as in claim 28 further including a shroud surrounding butspaced from the periphery of the development electrode, the mist beingretained in the space between the shroud and the development electrodeby means of an electric field between these members counteracting themomentum of the mist droplets travelling in the arcuate path.
 39. Anelectrostatic image development arrangement as in claim 28 wherein themeans to produce a mist is an ultrasonic transducer or a high pressurespray.
 40. An electrostatic image development arrangement as in claim 28wherein the mist has a droplet size of from 0.1 to 100 microns and. 41.An electrostatic image development arrangement as in claim 40 whereinthe mist has a droplet size of from 0.5 to 5 microns.
 42. Anelectrostatic image development arrangement as in claim 28 includingmeans to charge the mist to a positive or negative charge either at orimmediately after the time of production.
 43. An electrostatic imagedevelopment arrangement as in claim 28 further including an extractor toextract unused mist when the mist has passed the development station.44. An electrostatic image development arrangement as in claim 28further including a fan means to recycle the mist around the developmentdrum.
 45. An electrostatic image development arrangement as in claim 28wherein the colourant comprises a dye stuff or mixture of dye stuffs, apigment or mixture of pigments or a mixtures of dyestuffs and pigmentsso as to render the said latent electrostatic image visible whendeposited thereon.
 46. An electrostatic image development arrangement asin claim 28 wherein the colourant comprises a polymeric material whichupon deposition onto the recording member surface may be fixed such thatthe recording member may be used as a plate for offset printing withoutan image being visible thereon.
 47. An electrostatic image developmentarrangement as in claim 46 wherein the colourant further includesbactericides, humectants, dispersants, fixing agents, binders and chargecontrol agents such that all of these may be supported in the mist andbe carried with the mist to deposit on the electrostatic image.
 48. Anelectrostatic image development arrangement for high speed printingcomprising;(a) mist production means to produce a mist from a liquidcomprised substantially of water with a colourant incorporated thereinsuch that the colourant is supported in the mist, (b) means to transportthe mist in an arcuate path to a development station, (c) classificationmeans to select mist droplets of a selected size range and to discarddroplets above and below the selected size range, (d) means to transporta recording member having a latent electrostatic image thereon throughthe development station, (e) the means to transport the mist furthertransporting the mist of the selected size range across the recordingmember between the recording member and an adjacent developmentelectrode such that its direction of travel is substantially parallel ortangential to the recording member, (f) means to apply an electric fieldbetween the development electrode and the recording member whereby themist is attracted to the electrostatic image to instantaneously developit, and (g) means to extract unused mist from the development station.49. A method of developing a latent electrostatic image for high speedprinting comprising the steps of;(a) producing a mist from a liquidcomprised substantially of water with a colourant incorporated thereinsuch that the colourant is supported in the mist, (b) transporting themist to a development station in an arcuate path, (c) classifying thremist as it is transported to the development station to select mistdroplets of a selected size range and to discard droplets above andbelow the selected size range, (d) passing the mist between adevelopment electrode and a recording member having a surfaceincorporating the electrostatic image at the development station suchthat the direction of travel of the mist is substantially parallel ortangential to the surface of the recording member, (e) providing anelectric field between the development electrode and the recordingmember, (f) attracting the mist by means of the electric field to theelectrostatic image to instantaneously develop the electrostatic image,and (g) extracting the residual unused mist from the developmentstation.
 50. A method of developing an electrostatic image using a mistcomprising substantially water with a colourant incorporated thereinsuch that the colourant is supported in the mist including the step ofclassifying the mist into a selected droplet size range beforedevelopment of the image.
 51. An electrostatic image developmentarrangement for high speed printing including mist production means toproduce a mist from a liquid comprised substantially of water with acolourant incorporated therein such that the colourant is supported inthe mist and classification means to separate droplets from the mistwhich are above or below a selected size range before development of anelectrostatic image.